The present invention relates generally to a transconductive amplifier, and more particularly to, a current controlled transconductive amplifier.
As shown in FIG. 1, with an input voltage Vi a traditional transconductive amplifier has an output current Io in a relation
Io=gmxc3x97Vi,xe2x80x83xe2x80x83[Eq-1]
where gm is the transconductance of the transconductive amplifier T1.
In a general application, such as a GM-C filter, the transconductance thereof is controlled by an input voltage and is not necessary to have a linear relation with the input voltage. While in some applications, such as a telephone, the transconductance thereof is controlled by the line current and requires a linear relation to the line current. As shown in FIG. 2, the prior art constructs a transconductive amplifier with a differential stage composed of bipolar transistors. The transconductance of a bipolar transistor is proportional to the current flowing through the bipolar transistor, as a result, linear adjustment of the current flowing through the bipolar transistor can linearly control the transconductance of the bipolar transistor, and thus requirement of linear control of the transconductance of a transconductive amplifier can be met.
However, most present integrated circuits (ICs) are formed by metal-oxide-semiconductor (MOS) processes. In consideration of the consistence of manufacture process and circuit, it is obviously that the circuit shown in FIG. 2 is unsuitable to such circumstances. Even though the integration of bipolar transistor and MOS process is available, the manufacture processes will become complex and costly. It is thus desired a MOS transconductive amplifier.
Unfortunately, there is a problem presented in the MOS transconductive amplifier for that the transconductance of a MOS transistor and the current flowing through the MOS transistor have the relation
xe2x80x83gm=dId/dVgs=2Kp(W/L)(Vgsxe2x88x92Vt)=2(Kp(W/L)Id)xc2xd.xe2x80x83xe2x80x83[Eq-2]
That is, the transconductance of a MOS transistor is proportional to the square root of the current flowing through the MOS transistor. Therefore, it cannot obtain a circuit with linear control of the transconductance of a MOS transconductive amplifier by current as that does in FIG. 2.
A CMOS transconductive amplifier arrangement, according to the present invention, comprises a control circuit to provide a current to linearly control a first transconductive amplifier having a differential pair composed of a first and a second MOS transistors biased with a bias current, and a second transconductive amplifier matched to the first transconductive amplifier to provide an output coupled to an error amplifier and thus to form a feedback circuit to generate a current to provide the bias current for the second transconductive amplifier whose two inputs are coupled to a resistor therebetween flowing with a current provided by a first current source and output is coupled to a second current source with a current mirrored from the current of the feedback circuit by a current mirror for the bias current of the first transconductive amplifier so as to linearly control the transconductance of the first transconductive amplifier by a current.